Optical-based recording systems such as compact-disk read only memory (CD ROM), video disk, write once read many (WORM), etc., are gaining wide acceptance in the marketplace because of their high storage capacity, random access capability, and relatively low cost. Moreover, erasable optical data storage systems (e.g., magneto-optical disk drives) are now commercially available, wherein old data stored on the optical media can be erased and new data written in place thereof.
The basic principle underlying typical optical recording systems involves the use of a light beam to write data to and read data from an optical medium. Typically, data is represented as a series of digital bits. A light source, such as a laser, is focused and directed by a lens assembly onto a circular optical disk. The disk is rotated about a spindle. By pulsing the laser, data is stored digitally onto the spinning optical disk. Subsequently, the stored data is read from the optical disk by detecting the light beam reflected from the optical disk.
A servo mechanism is used to locate the light beam in reference to fixed radial locations over the disk surface. Hence, the light beam is repositioned among numerous radial positions. As the servo mechanism sequentially moves the light beam radially across the spinning optical disk, a spiral, nearly-concentric circular groove, referred to as a "track," is described on the optical disk. It is onto the surfaces of these tracks that digital information is stored.
Accessing a different track involves utilizing the servo mechanism to position the lens assembly so that the beam is focused onto the target track. This process is known as a "seek." Often, data is written in a random manner. Consequently, a number of seeks are performed for a single disk access. Since data cannot be written onto nor retrieved from the disk when a seek is in progress, it is important to complete the seek as fast as possible in order to minimize undue delays. Furthermore, given that the tracks are approximately one to two microns in width, it is imperative for the seek to be very precise. Otherwise, the seek would fail.
In many prior art optical recording systems, the laser and the detector along with the lens assembly are moved as a package when performing a seek. Although this simple, straightforward seek method is precise, it is extremely slow. The optics package constitutes a relatively large mass which renders it rather difficult to accelerate and decelerate quickly. In other words, it has a relative long seek time due to its weight. Moreover, moving such a heavy optics package entails excessive power consumption. For portable, battery-operated systems, conserving power is critical.
One method for reducing the payload of the actuator involves replacing the focusing motor with a focusing relay lens actuator situated in the fixed optics area. Similarly, the tracking function can be accomplished by utilizing a tracking relay lens actuator also situated in the fixed optics area. In regards to the focus relay lens, the longitudinal motion of the relay lens is optically de-magnified by the image lens relative to the square of the focal length ratio. As regarding the tracking relay lens, the lateral de-magnification is linearly proportional to the focal length ratio. This means that the actuators would be required to handle extremely high levels of acceleration in order to keep up with a relatively fast seek. With present lens actuator technology, the desired level of acceleration is extremely difficult if not impossible.
Another method for achieving a faster seek time involves implementing a plane mirror mounted on a galvo motor. As the galvo is pivoted, this causes the plane mirror to likewise pivot. Pivoting the plane mirror reflects the light beam at different angles. For fast seeking, a galvo fine tracking motor is implemented to maintain the light beam to the center of each track from take-off to landing. A coarse servo system for moving the lens is slaved to the movement of the light beam. Thus, in this approach, rather than moving the entire optics package, the laser, detectors and galvo remain fixed. Only the lens assembly are moved, resulting in a faster seek.
However, one serious drawback of this prior art seek scheme is that an offset is induced during the seek. When the seek is in the stages of take-off, switch-over, and landing, the coil currents for the linear coarse actuator and the galvo fine tracking motor have different rise time. This difference creates a momentary mismatch in their accelerations. Since the coarse actuator reacts slower than the fine tracking actuator, the principle ray of the laser beam is displaced from the optical axis of the objective lens. In turn, this causes the light spot reflected from the disk to be misaligned in relation to the tracking detector. Unfortunately, this develops an offset in the tracking error signal.
FIG. 1 shows a ray trace of a prior art galvo actuator system. The light beam from the laser, as represented by the solid lines, are reflected from the galvo mirror 100 to routing mirror 101. The light beam reflected from routing mirror 101 hits the objective lens 102 which focuses the light beam onto media 103. Because the light beam squarely hits the objective lens 102, there is no offset. However, as the galvo 100' is rotated to the objective lens 102, the new path of the light beam, as represented by the dashed lines, partially misses the objective lens 102. The top view of objective lens 102 is shown as 102'. The light beam associated with the rotated galvo is shown as 104. Note the offset. Moreover, for a linear coarse actuator system, the amount of offset is not only a function of the acceleration mismatching, but is also a function of the optical path length. The tracking offset introduces an error is the seek algorithm and can eventually lead to a seek failure if the offset become excessive.
Thus, there is a need in prior art optical recording systems for performing fast, precise seeks without using a relay lens system without introducing an offset. It would also be preferable for such a system to have low power dissipation.